When I draft emails, I always remember the below mnemonic, I read in a magazine.

The ABC of writing – Accuracy, Brevity & Clarity.

At work, I learnt a similar mnemonic: KiSS – Keep it Short & Simple)
Also like this essay Write Simply by Paul Graham.
Remember: Audience first. What is the 1 core message you want the audience to remember?
I learnt to write short & simple emails. My blog posts are very lengthy. Gradually learning to make what I write short & simple.